For the purposes of this description, a crack is defined as any non-designed physical discontinuity and the term shaft encompasses any axially extending structure which has a length considerably larger than its cross sectional dimension. Such structures take a wide variety of forms and are employed as motor rotors, shafts of pumps, generators, compressors and turbines, bolts and other fasteners, piping, etc. Although the present invention is applicable to any such structures, it will be presented, by way of example, in the context of detecting a crack in a reactor coolant pump shaft of a pressurized water reactor (PWR). Other examples would include boiling water reactor (BWR) recirculation pumps.
Nuclear reactors have been operating and producing useful electricity for many years. Within the last few years, several plants have found cracks in the reactor coolant pump shaft near the thermal barrier.
The large reactor coolant pump of a PWR circulates water out of the reactor vessel into steam generators which in turn pass steam to a steam turbine. The reactor coolant pump system consists of a vertical pump with a vertical motor mounted on the pump from above. In a typical design, the entire shaft system hangs vertically and is supported by a thrust bearing located on the top of the vertical motor. The pump system usually has an overhung impeller and an axial suction inlet from below the pump. The cooling water exits the pump through a single radial discharge in the horizontal direction. A net radial force is developed on the rotating shaft during the operation of the pump. This unidirectional unbalanced force applied to the rotating pump shaft has lead to fatigue cracks in the shaft and subsequent pump shaft failure for some pump designs.
The consequences of an unforeseen pump shaft failure can be severe. A nuclear facility can lose millions of dollars a day in revenues from an unscheduled outage. Since pump shaft replacement is an expensive, time consuming project, it is highly desirable to be able to discover the crack condition early and thus have time to plan and schedule the replacement.
In co-assigned U.S. Pat. No. 4,975,855, issued Dec. 4, 1990 (Miller et al.), the entire contents of which are hereby incorporated by reference, the presence, size and location of a crack in a shaft is determined by comparing actual measured natural frequencies of the shaft with the results of an analytical model. From a multistation analytical model of an uncracked shaft, natural frequencies and associated mode shapes are derived. A suspected axial location of a crack is defined and a natural frequency of interest is selected which has an associated mode shape exhibiting significant localized bending at the suspected axial location of the crack and a site of response measurement. The analytical model is modified to include a representation of a radial crack at the probable axial location, and the predicted split and downward shift of the natural frequency of interest as a function of crack depth is calculated from the modified model. The actual shaft is subjected to a radial excitation force, and vibrational response measurements are taken with a single accelerometer diametrically opposite the input transducer. Measurements are taken sequentially along multiple radial directions. A fast Fourier transform analyzer derives a frequency response function from the measurements for each radial direction. The frequency response functions indicate the actual natural frequencies of the shaft. A comparison of these actual natural frequencies with those predicted by the modified model is employed to determine the presence and severity of a crack in the shaft.
Earlier collars mounted only one input and a diametrically opposed output transducer for radial testing. This required the test technician to loosen the collar and rotate the shaft, or if that was not possible, to rotate the collar and move the shaker position to establish a new radial excitation direction. The collar/shaft rotation should ideally be 45.degree. for each radial test direction. However, the access to the coupling on all of the pumps to date is very limited, allowing only a .+-.27.5.degree. rotation of the collar. The shaft systems on most major machines are too massive to allow for the rotation of the shaft. As a result of this limitation, only three radial directions were capable of being tested utilizing the previous collars. This left a rather large gap in the test data. Also, there have been tests where the test shaft was resting against a bearing in one of the test directions, making the data from that one direction almost useless.
A need thus exists for a means of putting energy into and measuring energy from the shaft under test at multiple locations around the entire circumference of the shaft. This means should be easy to install, make no permanent marks to the shaft, and allow for quick, efficient, and accurate recording of the input and output energies.